Lansoprazole is the generic name for 2-[[[3-methyl-4-(2,2,2-trifluoroethoxy)-2-pyridinyl]methyl]sulfinyl]-1H-benzimidazole and it has the following formula (I):

Lansoprazole is a well-known gastric proton pump inhibitor and thus it can inhibit gastric acid secretion and is used as an antiulcer agent.
Processes for preparing lansoprazole are known. Several methods involve the use of a lansoprazole precursor having a thioether group which is subjected to an oxidation.
EP 0 174 726 B1 discloses the use of a peracid such as m-chloroperbenzoic acid, sodium bromite, sodium hypochlorite or hydrogen peroxide as an oxidizing agent. The oxidation is carried out in halogenated hydrocarbons, amides, alcohols or mixtures thereof.
EP 0 302 720 A1 describes a process for preparing lansoprazole using hydrogen peroxide in the presence of a vanadium compound such as vanadium pentoxide, sodium metavanadate or vanadium acetylacetonate.
According to WO 02/062786 A1 lansoprazole is prepared by using tert-butyl hydroperoxide (TBHP) in the presence of vanadium pentoxide, sodium metavanadate or vanadium acetylacetonate. Preferably, the oxidation is performed in toluene or isopropanol.
WO 2004/011455 A1 discloses a process for preparing lansoprazole using tert-butyl hydroperoxide (TBHP) in the presence of vanadium oxytrichloride as catalyst wherein the reaction is carried out in a solvent such as a C1- to C5-alcohol, decane, nonane, toluene or a mixture with water. The vanadium oxytrichloride is used in the presence of an alcohol. Moreover, the reaction is preferably performed in the presence of a weak base. Crude lansoprazole is obtained in a yield of at most 90%.
WO 03/008406 A1 refers to a process for preparing benzimidazole-type compounds by reacting a corresponding precursor with an oxidizing agent in a suitable solvent, extracting the sulphone by-product and isolating the product. Preferably, m-chloroperbenzoic acid is used as oxidizing agent.
The processes according to the prior art for preparing lansoprazole suffer from a low yield of the produced lansoprazole as well as a high content of impurities of the lansoprazole. This may be due to the fact that the selectivity in the oxidation step is low and thus by-products such as the corresponding benzimidazole N-oxide and the corresponding sulphone can be formed by an oxidation of the nitrogen and an overoxidation of the sulfide, respectively. When scaling-up the processes according to the prior art, the control of impurities is even more difficult resulting in extensive isolation procedures in order to obtain pure lansoprazole.
Consequently, there is still a need for an improved process for preparing lansoprazole avoiding the afore-mentioned drawbacks. In addition to that it will be highly advantageous, if solid lansoprazole is provided which is quickly released from a pharmaceutical composition.
These problems are surprisingly solved by the present invention.
2C: pellets comprising lansoprazole having a BET-surface area of 0.87 m2/g (prepared according to Example 3),
16C: pellets comprising lansoprazole having a BET-surface area of 1.00 m2/g (prepared according to Example 5),
17C: pellets comprising lansoprazole having a BET-surface area of 4.67 m2/g (prepared according to Example 4).